The present invention relates to lithium secondary cells, i.e., to improvements in lithium secondary cells wherein the negative electrode is made chiefly from metallic lithium, lithium alloy and/or a carbon material or oxide material capable of absorbing and desorbing lithium, and the positive electrode is prepared mainly from a positive electrode material typical of which is a metallic oxide. More particularly the invention relates to improvements in the positive electrode terminal and the negative electrode terminal for delivering current from an electrode unit serving as the electricity generating element to an external circuit.
The negative electrode materials heretofore proposed for use in lithium secondary cells include graphite, coke and like carbon materials, metallic lithium, lithium alloys and tin oxides. Among these, carbon materials are already in use for negative electrodes to provide lithium secondary cells. Graphite is one of the materials which are generally used for negative electrodes because graphite exhibits a discharge potential in close proximity to the potential of metallic lithium to afford lithium secondary cells of high energy density.
For example, JP-A No. 92335/1997 discloses one of lithium secondary cells wherein such materials are used for the negative electrode. The proposed cell has a negative electrode prepared from a carbon material and a negative electrode output terminal made from pure copper. Pure copper remains stable at the negative electrode potential during the charging and discharging of the lithium secondary cell and is therefore used for the negative electrode output terminal. Besides pure copper, titanium, nickel, stainless steel, etc. appear useful as potentially stable materials, whereas pure copper is thought suitable in view of ease of working.
However, pure copper is susceptible to oxidation and liable to form an oxide film at the portion of the cell exposed to the atmosphere, so that when used for the negative electrode terminal, pure copper has the problem of giving increased contact resistance at the connection to an external circuit, causing faulty contact to result in a discharge voltage drop.
On the other hand, pure aluminum is used for the positive electrode terminal of such a lithium secondary cell (see, for example, JP-A No. 92335/1997) since pure aluminum is also stable at the positive electrode potential during the charging and discharging of the cell. Although titanium, stainless steel, etc. appear useful as potentially stable materials besides pure aluminum, pure aluminum is considered to be suitable from the viewpoint of easy of working, conductivity and material cost.
Pure aluminum is nevertheless prone to form an oxide film, so that when used for the positive electrode terminal, this metal has the problem of offering greater contact resistance at the connection to an external circuit, giving rise to faulty contact or causing a discharge voltage drop as in the case of the negative electrode terminal.
Moreover, the positive or negative electrode terminal is not always satisfactory in mechanical strength and is not always suitable to tighten up with sufficiently great torque when a lead is to be attached thereto for connection to an external power source. This entails the problem that the terminal mount portion will not be sealed off effectively.
An object of the present invention, which is to overcome these problems, is to propose improved positive electrode terminal and negative electrode terminal, and an improved electrode terminal for a positive or negative electrode, for use in delivering the electric energy produced by an electricity generating element to an external device, and to further provide a lithium secondary cell having the positive electrode terminal and/or the negative electrode terminal.
Another object of the invention is to use the positive electrode terminal and/or the negative electrode terminal to assure the terminal or terminals of an enhanced mechanical strength in fabricating the cell and thereby improve the reliability of electrical connection of the cell to an external circuit and give an improved sealing effect to the terminal mount portion or portions. The formation of oxide film on the surfaces of the positive and negative electrode terminals is inhibited, enabling the terminals to retain high conductivity to suppress the discharge voltage drop of the cell.
To fulfill the above objects, the present invention provides a lithium secondary cell which comprises an battery can 3, a positive electrode terminal 51, a negative electrode terminal 81, an electrode unit 4 and an insulating member 53, the battery can 3 having the electrode unit 4 housed therein, the electrode unit 4 having a positive electrode and a negative electrode which are electrically connected to the positive electrode terminal 51 and the negative electrode terminal 81, respectively, the electrode terminals 51 and 81 being insulated from each other by the insulating member 53. The lithium secondary cell is characterized in that the positive electrode terminal 51 is formed from an aluminum alloy containing at least 1.0 wt. % of a different metal as an additive element.
With the lithium secondary cell of the invention, the positive electrode terminal 51 has a remarkably improved strength and can therefore be tightened up with sufficiently great torque.
Stated more specifically, the different metal in the aluminum alloy can be at least one element selected from the group consisting of Mg, Si, Fe, Cu, Mn, Zn, Cr and B.
When the aluminum alloy contains at least 0.30 wt. % to not greater than 0.85 wt. % of Mg, reduced electric resistance will result, giving the cell an increased power density.
Reduced electric resistance and an increased cell power density are available alternatively when the aluminum alloy contains at least 0.25 wt. % to not greater than 0.75 wt. % of Si.
Further stated more specifically, the aluminum alloy has the composition of A6101 prescribed in JIS, i.e., a composition comprising 0.35 to 0.8 wt. % of Mg, 0.30 to 0.7 wt. % of Si, 0.50 wt. % of Fe, 0.10 wt. % of Cu, 0.03 wt. % of Mn, 0.10 wt. % of Zn, 0.03 wt. % of Cr, 0.06 wt. % of B and the balance Al.
The cell can be so constructed that the battery can 3 and the positive electrode terminal 51 are insulated from each other by the insulating member 53. Further the battery can 3 and the negative electrode terminal 81 can be insulated from each other by the insulating member 53. Additionally, the battery can 3 and the positive electrode terminal 51, as well as the battery can 3 and the negative electrode terminal 81, may be insulated from each other by the insulating member 53.
The present invention provides another lithium secondary cell which comprises an battery can 3, a positive electrode terminal 51, a negative electrode terminal 81, an electrode unit 4 and an insulating member 53, the battery can 3 having the electrode unit 4 housed therein, the electrode unit 4 having a positive electrode and a negative electrode which are electrically connected to the positive electrode terminal 51 and the negative electrode terminal 81, respectively, the electrode terminals 51 and 81 being insulated from each other by the insulating member 53. The lithium secondary cell is characterized in that the negative electrode terminal 81 is formed by plating a substrate of copper with nickel.
Most suitably, the substrate of the negative electrode terminal 81 is made of oxygen-free copper.
The present invention provides another lithium secondary cell which comprises an battery can 3, an electrode terminal 511, an electrode unit 4 and an insulating member 53, the battery can 3 having the electrode unit 4 housed therein, the electrode unit 4 having two electrodes electrically connected to the electrode terminal 511 and the battery can 3, respectively, the electrode terminal 511 and the battery can 3 being insulated from each other by the insulating member 53. When serving as the positive electrode terminal, the electrode terminal 511 is formed from an aluminum alloy containing at least 1.0 wt. % of a different metal as an additive element. Alternatively when serving as the negative electrode terminal, the electrode terminal 511 is formed by plating a substrate of copper with nickel.
With the lithium secondary cell described of the invention, the electrode terminal 511 has a remarkably improved strength and can therefore be tightened up with sufficiently great torque. Consequently, the terminal mount portion, i.e., the portion where the terminal is attached, is given an enhanced sealing effect.
Stated more specifically, the different metal in the aluminum alloy can be at least one element selected from the group consisting of Mg, Si, Fe, Cu, Mn, Zn, Cr and B.
When the aluminum alloy contains at least 0.30 wt. % to not greater than 0.85 wt. % of Mg, reduced electric resistance will result, giving the cell an increased power density.
Reduced electric resistance and an increased cell power density are available alternatively when the aluminum alloy contains at least 0.25 wt. % to not greater than 0.75 wt. % of Si.
Further stated more specifically, the aluminum alloy has the composition of A6101 prescribed in JIS, i.e., a composition comprising 0.35 to 0.8 wt. % of Mg, 0.30 to 0.7 wt. % of Si, 0.50 wt. % of Fe, 0.10 wt. % of Cu, 0.03 wt. % of Mn, 0.10 wt. % of Zn, 0.03 wt. % of Cr, 0.06 wt. % of B and the balance Al.
Further it is most suitable that the substrate of copper be oxygen-free copper.
Examples of materials usable for the negative electrode of the cell of the invention are graphite, coke and like carbon materials, metallic lithium, lithium alloys and tin oxides.
Examples of materials usable for the positive electrode of the cell of the invention are a wide variety of those which have heretofore been used in nonaqueous-type cells, such as lithium containing composite oxides (e.g., LiCoO2). Such a material is used as a kneaded mixture in combination with an electrically conductive agent, such as acetylene black or carbon black, and a binder, such as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVdF).
Further examples of solvents useful for forming the electrolyte are ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methylethyl carbonate, sulfolane, 3-methylsulfolane, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran and 1,3-dioxolane. These solvents are usable singly or in mixture. However, these examples are not limitative.
Examples of preferred electrolytes are generally those containing fluorine, such as lithium hexafluorophosphate, because these electrolytes are stable and advantageous from the viewpoint of discharge capacity and charge-discharge cycle characteristics. More specific examples of useful electrolytes are LiPF6, LiBF4, LiCF3SO3, LiAsF6, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, and at least one of mixtures of such compounds.
Examples of separators usable in the lithium secondary cell embodying the invention are a wide variety of those having high ionic conductivity and conventionally used in lithium secondary cells, such as finely porous membranes of polyethylene or polypropylene.